1. Field of the Invention
This invention relates to solder mask coatings having improved performance characteristics, and, more particularly, to aqueous alkaline developable, UV curable acrylate compounds and compositions useful for forming 100% solids, solvent-free, photoimageable liquid solder masks.
2. Description of the Prior Art
An article in "Circuits Manufacturing", pages 24-33 (January, 1987) describes the recent commercial advances in the art of photoimageable liquid solder masks, which are intended to provide technical and cost advantageous over traditional dry film solder masks.
A number of U.S. patents disclose photopolymerizable oligomers and compositions for use as solder masks in printed circuit boards, including U.S. Pat. Nos. 3,660,088; 3,753,720; 3,883,352; 3,887,450; 4,199,163; 4,228,232; 4,304,923; 4,361,640; 4,436,806; 4,458,007; 4,481,281; and 4,508,916. However, none of those UV curable compounds and compositions are entirely satisfactory for use as a commercial solder mask, particularly for a screen-printable, photoimageable, 100% solids, solvent-free liquid solder mask which exhibits advantageous performance characteristics, and is aqueous alkaline developable.
Rousseau, in U.S. Pat. No 4,228,232, describes an oligomer and composition for use in a presensitized plate for printing operations, such as is used in the formation of lithographic plates. For such application, the oligomers must be tough but do not require flexibility, and heat, chemical and electrical insulation resistance, high photoresponse, screen printability, adhesion to substrate, and other properties essential for solder mask operation. Accordingly, Rousseau's oligomer has a high molecular weight polymeric backbone derived from an aromatic diisocyanate and polymeric polyols, as polyester and polyalkylene polyols, which have molecular weights up to 10,000, and high hydroxy equivalent weights of 45 to 5,000, most preferably 200 to 2,000. The compositions formulated by Rousseau from these oligomers require an organic solvent to make the oligomers soluble.
Rendulic, in U.S. Pat. No. 4,436,806 describes a method and apparatus for making a printed circuit board by imaging a liquid polymer which has been coated onto the board by roller coating. In this process, imaging is carried out while the polymer is wet and the phototool is positioned in close proximity to the coated board. The liquid polymer compositions disclosed by Rendulic for use in this process are urethane acrylates having a polyether or polyester polyol chain extending unit. Such compositions have a very low viscosity, which is desirable for forming coatings with a roller coater. However, these compositions have a relatively low degree of ethylenic unsaturation per weight unit so that they require a relatively high energy level for curing.
In contrast to the imaging technique of the above mentioned patent, Sullivan, in U.S. Pat. No. 4,506,004, describes a contact method of imaging a liquid solder mask coating is screen printed to a given thickness, prehardened and imaged by exposure with suitable UV energy. The desired characteristics of excellent product performance and printing resolution are the attributes of the Sullivan system. The requirements of a suitable liquid photopolymerizable compound for use in this process include an ability to form a smooth, flexible coating which can impart chemical, electrical insulation and heat resistance to the solder mask. Furthermore, the coating must exhibit excellent adhesion to the printed circuit board with different types of metallization, and an ability to be cured to a desired depth with a minimum amount of UV energy.
Accordingly, an object of the invention is to provide a solder mask coating which can be developed with an aqueous, slightly alkaline solution, thereby avoiding the use of organic solvents in the developing step. The solder mask coatings also should exhibit flexibility, heat, chemical and electrical insulation resistance, surface hardness, abrasion resistance, adhesion to the underlying metal of the printed circuit board, and high cure depth at a low U.V. dosage. These stringent requirements imply that the structure of the prepolymer and its formulation must be carefully designed before the solder mask can hope to achieve commercial acceptability.
A further object of this invention to provide a liquid UV curable urethane acrylate compound, and a 100% solids, solvent-free composition for use as a solder mask, which composition can be developed rapidly in an aqueous, slightly alkaline solution, preferably at a rate of at least 1 mil/min., at temperature of less than 39.degree. C. and at a pH of 8-11, without leaving a residue.
Another object of this invention is to provide a UV curable urethane acrylate compound and composition having a suitable viscosity which enables coating by screen printing techniques to form a smooth, uniform, glossy, flexible coating of a thickness of 2 mils or more.
Still another object herein is to provide such a solder mask coating which can be cured, preferably at an energy level of less than 0.3 joules per sq. cm., to provide a cure depth of at least 18 mils at this energy level.
Yet another object of the invention is to provide a solder mask composition which is stable without gelation for at least six months, under ambient storage conditions.
A further object herein is to provide a flexible solder mask coating having excellent adhesion to the underlying metal of the printed circuit board, particularly on copper and tin-lead.
Another object herein is to provide a solder mask coating which exhibits good surface hardness, abrasion, heat, organic solvent and electrical insulation (Class III-IPC) resistance.
A specific object herein is to provide a composition which is particularly adapted for use in the method and apparatus of making printed circuit boards described in U.S. Pat. No. 4,506,004.
A further object of the invention is to provide a UV curable urethane acrylate compound and composition which meets acceptable environmental standards.